Digital imaging technology is being used in an increasing variety of mass-produced applications and in increasing production volumes. For example, miniature fixed-focus digital camera modules are being incorporated into end products such as portable telephones and personal digital assistants (PDA's). Given the new high volume applications for imaging modules, it is a continuing objective for imaging module fabricators to reduce their size and cost, and to provide imaging module designs that simplify incorporating the imaging modules into end products made by original equipment manufacturers (OEMs).
Imaging modules generally include an image sensor that detects an image and converts it into an electrical signal representation. An image processor is employed to further manipulate the image signal into an image of a suitable quality for output. Electrical contacts provide connectivity between the imaging modules and the end products incorporating the imaging modules.
One type of imaging module is commonly referred to as a camera module. Camera modules are known in the art as miniature complete camera assemblies for incorporation into end products. A camera module generally includes an image sensor, an image processor, interface circuitry and hardware, and a lens for focusing incoming light onto the image sensor. A conventional camera module also typically includes a chassis and/or enclosure for mounting the various electronic and optical components and for protecting the components from particulate and spurious light contamination. A typical chassis/enclosure includes focusing features for aligning the lens and image sensor during fabrication. The focusing features are commonly in the form of mating pieces as part of the main chassis/enclosure, and lens assembly piece. During fabrication, the mating pieces are specially aligned so that the lens directs an image onto the image sensor. This assembly step is typically performed on each individual camera module fabricated. Therefore, the assembly step is time consuming and labor-intensive, and therefore costly.
End products that incorporate imaging modules include a mechanism for mechanically and electrically attaching the imaging modules to the host devices. Conventional mechanisms for attaching the imaging modules to the host devices tend to be costly when compared to the costs of attaching other types of electronic/electrical hardware, such as integrated circuits (IC's). Conventional imaging modules are not suitable for automatic assembly. A conventional imaging module is typically located on a circuit board that is enclosed by the host device's housing. The imaging module is secured to the circuit board with adhesive, mounting hardware, or structural features. In other types of conventional designs, the imaging module attaches mechanically to the housing of the host device. In these examples, an electrical connection between the imaging module and the host device is typically accomplished with a multi-conductor cable and mating electrical connectors on the circuit board. Such features are expensive, not well suited for automatic assembly, bulky, and less reliable than soldered connections.
Standard automatic electronic assembly processes are not readily adaptable to support automatic mounting and connecting of conventional imaging modules. Thus, assembly of host devices involves manual operations to mount and electrically connect the imaging modules. For example, conventional digital camera modules are mounted to the host device by hand, and the connector is inserted by hand. By nature, manual operations are slow, labor-intensive, and prone to error; therefore, manual assembly steps are expensive. Furthermore, the material costs of conventional imaging modules include the costs of the mounting and connecting hardware. Moreover, the space within the host device needed to accommodate conventional imaging modules includes space for the mounting and connecting mechanism. Additional space requirements inhibit host device miniaturization, cost reduction, and marketability.
By contrast, electrical/electronic hardware such as integrate circuits is typically assembled onto a host device's circuit board automatically. Most commonly, surface-mount technology (SMT) is employed, where each component is robotically placed and automatically soldered onto the circuit board using solder wave or re-flow processing. The soldered joints accomplish a mechanical and electrical attachment.
In a typical SMT process, a circuit board is first prepared to accept solderable components. Most commonly, solder paste is applied selectively to electrical pads that accept contacts of the electrical components to be mounted thereupon. Next, the electrical components are automatically placed into their appropriate positions on the circuit boards using robotics. This process is referred to as pick-and-place, or onsertion. The electrical components are temporarily held in place on the circuit board pads by the solder paste's adhesive properties. Next, the circuit board with components positioned thereupon is subjected to a solder re-flow process in order to create permanent solder joints at all electrical contact/pad interfaces.
A solder re-flow process involves heating the solder paste to a temperature where the solder melts and flows thoroughly onto both the mounting pads and electrical component contacts. The temperature of the solder is then allowed to drop, causing the solder to solidify into solder joints that mechanically and electrically bond the components to the circuit board. The temperature at the molten solder joints during the re-flow process is on the order of 250° C. Such temperatures can be reached by a variety of means, including conductive, convective, or radiant heating of the solder joint sites.
Components for use on circuit boards assembled using a re-flow process must be able to withstand the process temperatures. However, as with any manufactured product in mass production, conventional digital camera modules are constructed from low-cost materials whenever possible. Enclosure materials and lens materials are often made from various plastics. Although certain plastic materials can withstand re-flow temperatures for short periods of time, presently available materials are not suitable as lens material, which must have particular optical characteristics.
Recent advances in the art have achieved fully integrated imaging modules. These modules are essentially fully packaged integrated circuits having an optically transparent window that allows the image sensor IC to capture an image. In camera module applications, such imaging modules reduce cost, parts count and camera module assembly costs with their smaller size and increased level of integration. Still, camera modules incorporating these imaging modules use separate housings and lens assemblies, and lens alignment procedures as part of their manufacture. To date, no practical solution has been proposed for a fully integrated camera module, or a low-cost imaging module that can easily be assembled into a camera module.